Differences

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--- nsw_lidar [2021/02/17 08:43] – bushwalking
+++ nsw_lidar [2021/02/20 09:52] – bushwalking
@@ Line 1: / Line 1: @@
-====== Intro ======
+====== Mapping from NSW Lidar ======
 NSW Spatial Services have undertaken a program to map all of NSW using lidar (light detecting and ranging)
 For details, see information on their [[http://spatialservices.finance.nsw.gov.au/mapping_and_imagery/environmental_spatial_programs|elevation program]].
@@ Line 8: / Line 8: @@
 ====== Topics ======
-The original topics here have been moved to their own pages
+The original topics here are being progressively moved to their own pages
 ===== Managing DEMs =====
@@ Line 19: / Line 20: @@
 There are several primary data items for topographic maps that can be generated using the DEM data from the NSW Lidar.
-  * Hydrologically correct DEM - prior to further operations, it is usually important to make sure that your DEM is free of depressions. Otherwise these will mess up streams and possibly contours.
+  * [[qgis_depressionless_dem|Hydrologically correct DEM]] - prior to further operations, it is usually important to make sure that your DEM is free of depressions. Otherwise these will mess up streams and possibly contours.
 Once you have a depressionless DEM, the following items can be generated:
-  * Contours
+  * [[qgis_contours|Contours]]
-  * Hydrology (Stream Network)
+  * [[qgis_hydrology|Hydrology (Stream Network)]]
+  * [[qgis_slope|Slopes]] - while not a standard feature of topographic maps, this can be a simple way to identify cliffs
   * Clifflines
-  *
-====== Topographic maps ======
-There are several primary data items for topographic maps that can be generated using the DEM data from the NSW Lidar. The main ones are:
+===== Styles =====
-  * Contours
-  * Hydrology (Stream Network)
-  * Clifflines
-The steps below are works in progress to determine effective (the best?) ways to extract the various items out of the DEM data for use in topographic maps. Any feedback/suggestions of improvements are welcome.
+  * [[qgis_styling|Styling]] - download basic style files
-===== Fill Sinks =====
+===== Automation =====
-From the initial DEM, first step is to Fill Sinks. Otherwise you will get sinks in the middle of watercourses, which will impact contours and stream networks. Note that the this approach needs to be used with care in areas where there are actual depressions.
+  * [[qgis_basic_automation|Basic map creation]] - a set of PyQGIS scripts that will create (and optionally save) a basic topographic map. This map can be used in QField
-There are various related tools in the Processing Toolbox that will do this, including:
+===== Mobile apps =====
-  * SAGA : Terrain Analysis - Hydrology : Fill Sinks
-  * SAGA : Terrain Analysis - Hydrology : Fill Sinks (Wang and Liu)
-  * SAGA : Terrain Analysis - Hydrology : Fill Sinks XXL (Wang and Liu)
-The results from all will be similar, but the Wang and Liu versions should be faster.
+  * [[qgis_qfield|QField]] - QField is an excellent Android app for for viewing projects created with QGIS
-There are other approaches that deepen channels rather than fill sinks in order to get a hydrologically sound drainage network. For example
+====== Topographic maps ======
-  * SAGA : Terrain Analysis - Hydrology : Sink Removal
-has an option for this.
-===== Contours =====
+There are several primary data items for topographic maps that can be generated using the DEM data from the NSW Lidar. The main ones are:
-==== Basic Processing ====
+  * Contours
-There are various contour extraction algorithms in QGIS, for example:
+  * Hydrology (Stream Network)
-  * GDAL : Raster Extraction : Contour (same as Raster -> Extraction -> Contour...)
+  * Clifflines
-Below is an example of contours created without and with sink removal. The contours on the right have been derived from a DEM where the sinks (in yellow on the left) have been filled.
-{{:2019_02_08_12_17_09_untitled_project_qgis.png?300|}}
-{{:2019_02_08_12_17_57_untitled_project_qgis.png?300|}}
-Even with sink removal, small
-==== Simplifying ====
-Vectors can be compressed by using something like:
-  * Vector geometry : Simplify
-A tolerance of 1(m) seems reasonable for 1:25000 mapping. Smaller tolerances may be appropriate for larger scale maps (eg 1:10000, 1:5000).
-For more options in compression, look at:
-  * GRASS : [[https://grasswiki.osgeo.org/wiki/V.generalize_tutorial|v.generalize]]
-V.generalize can also be used to smooth contours - possibly best done prior to simplificiation
-==== Cleaning ====
-Once simplified, it is worth removing small closed loops, such as those in the image below.
-{{:contour_loops.png|}}
-Here is one approach, which involves adding a length attribute to each contour, and removing those that fall below a certain length. It may cause issues if you have short sections of contour near the edge of the map that you need.
-  * Open Attribute Table (F6)
-  * Open field calculator (Ctrl+I)
-  * Add new attribute length, calculated as $length
-{{::qgis_add_field.png|}}
-  * Select all features and filter on length < 25 (or whatever length is appropriate for your scale)
-{{:qgis_filter_field.png|}}
-==== Contour Labelling ====
-See separate page on [[[qgis_contour_labelling|QGIS Contour Labelling]]
-===== Hydrology (Stream Network) =====
-The starting point for hydrology is a hydrologically sound DEM, as above. Use a fill sinks or channel deepening algorithm.
-==== Catchment Areas ====
-Next step is to create Catchment Areas. Again, there is a Catchment Area tool (in fact several), and six methods within the tool. For the purpose of delineating watercourses in steep terrain, the choice of method probably makes little difference.
-  * SAGA : Terrain Analysis - Hydrology : Catchment Area
-This gives an output that is best viewed in log scale. You can do this via
-  * Raster -> Raster Calculator...
-    *  log10 ( "Filled DEM@1" )
-Use the log scale version to determine the cutoff for what streams you want to see and which ones are too small. 10000 seems to give comparable results to the existing 1:25000 maps.
-Note that if you don't have the entirety of the catchment, you may get erroneous results.
-==== Channel Network ====
-The following tool can be used to create channels (streams) - there are other options:
-  * SAGA : Terrain Analysis - Channels : Channel Network
-Use
-  * Elevation = Filled DEM
-  * Initiation Grid = Catchment Area
-  * Initiation Type = Greater Than
-  * Initiation Threshold = 10000 (or whatever number you have determined)
-{{:2019-02-08_12_41_50-channel_network.png?600|}}
-==== Classification ====
-For 1:25000 maps, I've had reasonable results from using the following formula in the Raster Calculator to classify the streams into categories. Different scales may need different bounds, and this doesn't account for significantly larger rivers.
-''( log10 ( "Catchment Area@1" ) >= x) * ( log10 ( "Catchment Area@1" ) < y) * ("Channel Network@1" != 0)''
-  * Intermittent: 4-6.15 (x-y)
-  * Minor: 6.15-7.4
-  * Major: 7.4+
-==== Convert to Vector and Simplify ====
-Convert to vector using r.to.vect
-{{:qgis_raw_stream.png?600|}}
-The raw stream data is very jagged. Smooth using
-  * v.generalize
-  * Algorithm = Hermite (there are other options which can be used, but Hermite has the smoothed line passing through the points of the original)
-  * Maximal tolerance value = 20 (in m, obviously scale dependent)
-Simplify using using:
-  * Vector geometry : Simplify
-Tolerance:?
+The steps below are works in progress to determine effective (the best?) ways to extract the various items out of the DEM data for use in topographic maps. Any feedback/suggestions of improvements are welcome.
 ===== Clifflines =====